1. Field of the Invention
The present invention relates to an apparatus for and a method of generating pointers for identifying structure boundaries of data for leased lines such as T1/E1 in an asynchronous transfer mode (ATM) adaptation layer Type 1 (AAL1) through an ATM layer. In particular, the present invention relates to an apparatus for and a method of generating pointers in an AAL1 for converting structured user data of narrowband-integrated service digital network (N-ISDN) and N×64 Kbps services in a broadband access subscriber network.
2. Description of the Related Art
Generally, an ATM adaptation layer (AAL) serves to perform functions for adapting services provided by an ATM layer to the request of the user of a higher layer.
The matching of both the higher layer and the ATM layer with the AAL is enabled through service access points between the convergence sublayer (CS) of the AAL and the higher layer and between the segmentation and reassembly sublayer (SAR) of the AAL and the ATM layer, respectively.
The AAL conducts mapping of the protocol data unit (PDU) of the higher layer with the payload slot of an ATM cell while processing transfer errors, and providing functions for performing processing for lost cells and inserted cells, flow control, and timing control. The AAL includes two sublayers, that is, the CS and the SAR.
The CS performs functions associated with specific services, whereas the SAR performs functions having no relation with completion of services, that is, functions associated with segmentation and reassembly of user information. At the transmission end, the CS receives user information from an upper layer, affixes a header to the user information to form a CS-PDU, and sends the CS-PDU to the SAR. The SAR cuts the CS-PDU to have an ATM cell size, affixes a header to the resultant CS-PDU to form an SAR-PDU, and transfers the SAR-PDU to the ATM layer. The headers affixed to the information passing through respective sublayers during the transfer procedure are associated with processing of errors, management of buffers, and keeping of sequences.
ITU-T, the telecommunication standardization sector of the International Telecommunications Union (ITU), established an ATM service standard for constant bit rate (CBR), real time, and connection, that is, Recommendation I363.1, in August, 1996. The ATM Forum established a circuit emulation service (CES) standard, that is, Recommendation af-vtoa-0078-000, in January, 1997. Services for users supported by the AAL1 include CBR data transfer services, timing information transfer services, user data structure information transfer services, error recovery services, cell delay variation processing services, and services for processing loss of cells and erroneous insertion of cells.
One conventional technique associated with generation of pointers for structured data in the AAL1 is a technique “APPARATUS FOR AND METHOD OF GENERATING POINTERS FOR TRANSFER OF STRUCTURED DATA IN AAL1” disclosed in Korean Patent No. 10-0185861 (Dec. 28, 1998) registered in the name of Daewoo Electronic Company, Ltd. This technique has proposed a pointer generating apparatus which includes a counter for counting user data to calculate the offset field of a pointer, a first register for outputting a count bit value corresponding to double the counted value from the counter, a second register for outputting a count bit value corresponding to a value obtained by adding “1” to the double value of the counted value from the counter, a multiplexer, other control circuits, and buffers. In accordance with this technique, however, the higher layer cannot support multiframe E1 or T1 time division multiplexing (TDM) trunks, in particular, N-ISDN channels. Also, when two or more ATM cells are generated for each N-ISDN channel, it is impossible to generate pointers for a plurality of virtual channels (VCs). Furthermore, there is a problem in that the pointer adapted to indicate the structure boundary of structured user data has a degraded accuracy.
Conventional AAL1 devices mainly use an unstructured data transfer mode to transfer data for leased lines such as T1/E1. Such leased lines, for example, T1/E1, form a 64-Kbps DSO time slot based on a voice channel into an 8-Khz frame. Accordingly, one frame for the T1 consists of 24 time slots, whereas one frame for the E1 consists of 32 time slots. One extended super frame (ESF) for the T1 consists of 24 frames. One multiframe for the E1 consists of 16 frames.
The above mentioned conventional technique has proposed a configuration including elements having only the functions capable of designing an apparatus for generating pointers of a simple format for user data structured to have a simplest format. However, user data structured to have diverse formats has recently been required to meet new multimedia services. In addition, it has been required to take into consideration the bandwidth not used in the procedure of converting data into ATM cells.
It is impossible to efficiently generate pointers capable of supporting user data structured to have diverse formats, only using the functional elements of the conventional technique.